Special Issue Preface pubs.acs.org/JPCA
Stereodynamics of Chemical Reactions 2012
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development of attosecond methods, which should track the movements of the electrons. Thus, the following subjects have been emphasized in this edition: − Through the observation of attosecond pulse generation in molecular nitrogen, Dr. Thierry Ruchon et al. (Saclay) could detect the subfemtosecond evolution of autoionizing molecular states in this molecule, paving the way for future experiments. − Electronic wave packets are a fascinating and new domain for time domain experiments as proposed by Pr. R. D. Levine et al. (Jerusalem). This leads the way for their observation in simple systems. − Pr. H. J. Loesch (Bielefeld) reviewed oriented reactions via the brute force orientation of polar molecules under a strong static electric field. Pr. Loesch is the recipient of the R.B. Bernstein award. Concerning oriented reactions, Kasai et al. reported a very large steric effect on the Xe-(3P) + CF3Br system with the Br end having the largest opacity function. − The stereodynamics of photoionization has been promoted by Dr. Danielle Doweck et al. (Orsay) by molecular frame photoemission. There, through coincidence with axial recoiling ions, the long sought molecular frame electron distribution can be obtained. This represents a sensitive probe of electronic and nuclear dynamics and a tool for molecular orbital reconstruction. − Pr. R. Wester et al. (Innsbrück) studied differential cross sections in SN2 reactive collisions of molecular ions and neutrals, typical of solution organic chemistry. This reveals under isolated conditions specific angle- and energy-dependent mechanisms otherwise inaccessible. Furthermore, under the influence of microsolvation with as few as one or two water molecules, the reaction dynamics of a nucleophilic substitution reaction is shown to change remarkably. − Scattering resonances are a delicate probe of potential energy surfaces, elusive to detect and characterize. Pr. Xueming Yang et al. (Dalian), in studying the details of the F + D2 chemical reaction dynamics, has characterized experimentally and theoretically a variety of dynamical situations including scattering resonances. In this respect, cold collisions are revealing resonances, as in the work of S. Chefdeville (S + HD), who was awarded the poster prize. − Transition-state spectroscopy is the heart of chemical dynamics and continues to develop. Pr. D. N. Neumark (Berkeley) showed unprecedented 5 cm−1 resolution in probing the electron photodetachment in FH2− and FCH4−, leading to products. This allows mapping with high resolution of molecular movements in the transition region. − Chirality is an essential feature for deciphering biological mechanisms, but circular dichroism generates only small signals. Pr. Ivan Powis (Nottingham) showed that the chiral scattering term of the photoejected electron in ionization experiments of chiral molecules or their clusters can depend
his section gathers significant papers from the XIVth Stereodynamics conference held in Paris from October 22 to 26, 2012. Regioselective chemistry and stereochemistry, familiar to chemists address the results of steric forces in chemistry through the formation of stereospecific products. Here, it is the direct observation of anisotropic forces, while they develop and lead to these products, that has been investigated and reported. Rafael Levine, one of the founders of the field of reaction dynamics that studies the forces driving chemical reactions, initiated with Dudley Herschbach, Richard B. Bernstein, and Steven Stolte a seminar on the Stereodynamics of Chemical Reactions in 1986 in Jerusalem. There, a deeper understanding was sought on the steric aspects of chemical reactions in the perspective of reaction dynamics, where the angle-dependent forces driving reactions are characterized. A handful of scientists participated in this memorial inauguration of, now, a series of conferences. The seminar had theoretical aspects with the exploration of potential energy surfaces and the modeling of results, but looking back, the most surprising was the emergence of classical mechanics methods with molecular mechanics and classical trajectories as valuable tools. Experimentally, photodissociation and vector correlations to disentangle the forces in the transition region were at the topmost of methods for unimolecular reactions. For bimolecular reactions, molecular orientation of the reagents via multipole fields or the “brute force” of a strong static electric field in crossed beam experiments was the privileged way to describe the angle-dependent forces in the recoil of products. Also, bimolecular reactions were made unimolecular by condensation of the adducts in a single complex. This allowed preorientation of the reagents and also transition-state spectroscopy of the bimolecular reaction. This meeting has gradually evolved from its inception with the study of elementary, model chemical systems to establish the concepts, toward more complex systems, with a complexity closer to that of systems of general interest in chemistry or biomimetic systems. There, it is crucial to unravel the nature of the stereospecific forces to guide the systems toward specific products. Since 1987, great experimental developments have been made with the characterization of the alignment and orientation of the products, velocity map imaging, directly in the molecular frame. Also, with the development of femtosecond lasers, molecular movements can be seen as they proceed. The idea of the active control of chemical reactions by tailored light fields is now a reality. The greatest leap forward is however the outburst of powerful and tractable quantum chemistry to predict and model reaction dynamics in groundstate systems. This should now be transferred to electronically excited systems. Up to now, stereodynamics have been explored through the movement of the nuclei. The nuclei, however, in chemistry are driven by the changes of electronic distributions throughout the reaction. Future developments will directly explore the changes in electronic distributions. This is underway through the © 2013 American Chemical Society
Special Issue: Stereodynamics Symposium Published: July 11, 2013 8093
dx.doi.org/10.1021/jp406560k | J. Phys. Chem. A 2013, 117, 8093−8094
The Journal of Physical Chemistry A
Special Issue Preface
sensitively on both the static configuration and conformation of the molecule; hence, it is a new and very sensitive probe of electronic distributions. − Pr. Dwayne Miller (Hamburg and Toronto) showed examples of the observation of molecular movement, revealing, as it proceeds, the ring closure in the cyclization reaction of diarylethenes, using time-resolved electron diffraction in organized media, which could be a technique of the future through both its structural and time resolution. In this respect, new media for the observation of reaction dynamics as liquid helium clusters were highlighted by Pr. Gary Douberly investigating radical−radical reactions. The future of this domain opens through many of the subjects presented, where very low energy collisions were characterized (Dr. M. Lepers, Orsay); the observation of large gas-phase biomimetic molecules will inspire the dynamics in biomolecules and the helium droplet cluster medium infrared structural measurements. Sub-angstrom distance resolution and femtosecond time resolution in crystals probed by ultrafast electron diffraction will initiate a series of a new views of the reactivity of space-oriented molecules. The observation of electronic wavepackets in molecules is also triggered by inspiring calculations (Pr. Levine, Pr. F. Remacle).
B. Carré L. Poisson N. Shafizadeh B. Soep
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dx.doi.org/10.1021/jp406560k | J. Phys. Chem. A 2013, 117, 8093−8094
